Field of the Invention
The invention concerns a method for recording a magnetic resonance data set of a target region of an examination object, especially of a patient, identified in each case by at least one parameter value of at least one material parameter in different image elements, with a magnetic resonance scanner. The method is of the type wherein, in a series of establishing steps, initially within the framework of a magnetic resonance sequence, magnetic resonance signals of a measurement region are recorded and thereafter, to establish each parameter value, the magnetic resonance signal of the respective image elements is compared with comparison signals specific for the magnetic resonance sequence, to which in each case assignment values of the at least one material parameter are assigned such that overall a range of values of the at least one material parameter in a defined resolution will be covered. The assignment value of the at least one material parameter, which is assigned to the comparison signal with the closest match, is used as the parameter value. The invention also concerns a magnetic resonance apparatus and an electronically readable data storage medium for implementing such a method.
Description of the Prior Art
Magnetic resonance imaging is a widely known imaging modality that is frequently employed in the medical context, in which a patient is the examination object. Such imaging is known as qualitative imaging, the aim of which is to generate a magnetic resonance image data set, which shows anatomical structures, in particular tissue, in order to be able to make diagnostic conclusions based hereon, about the state of health of a patient. Options for what is known as quantitative magnetic resonance imaging have also been proposed. Here quantitative material parameters, which can be measured by the magnetic resonance, are established, so that the magnetic resonance data sets produced of a target region to be recorded do not necessarily contain image values in the image elements of the magnetic resonance data set, but contain parameter values of at least one material parameter, so that the magnetic resonance data set can ultimately also be understood as a type of parameter map of the target region. Examples of material parameters determined in this way are the proton density and relaxation times, in particular the T1 relaxation time, the T2 relaxation time and the T2* relaxation time. The use of varying spatial resolution is known both for quantitative and also for qualitative magnetic resonance imaging. Thus an overview image of the target region can first be recorded, in which a subregion of interest can then be selected automatically and/or manually, for which new magnetic resonance data are recorded with a higher spatial resolution, in order, for example, to be able to present the region around a suspected lesion in a higher spatial resolution.
A proposed approach for quantitative magnetic resonance imaging, i.e. the determination of parameter values for at least one material parameter, is known as magnetic resonance fingerprinting as described, for example, the article by D. Ma et al., “Magnetic Resonance Fingerprinting”, Nature 495:187-192 (2013). In this article, instead of the usual magnetic resonance sequences, specific, pseudo-randomized magnetic resonance sequences with, for example, specific sequences of excitations, are used in order to induce a magnetic resonance signal of different materials, in particular different tissue, which is unique and thus represents a type of fingerprint, which in turn is a function of the material parameters or material characteristics that are to be discovered. In other words specific magnetic resonance sequences, usually containing a number of excitation pulses, are used in order to record magnetic resonance signals for image elements, which can then be identified as a specific fingerprint, by comparison, in particular correlation, with a library of comparison signals. The comparison signal that has the highest correlation is then given assignment values for the different material parameters, which can then be employed as the parameter values of the material parameters for the magnetic resonance data set.
In this case the assignment values cover specific ranges of values of the material parameters in a specific resolution, which serve as the basis for determining the combinations of values of the material parameters that have a sufficient, reliable ability to distinguish between the comparison signals themselves, and thus on comparison with the magnetic resonance signal. This allows an assignment that is as robust as possible, ideally a unique assignment, is to be made and ambiguities are avoided. In practice, it has been shown, however, that the number of the material parameter value combinations that can be distinguished by a magnetic resonance signal to be created for a specific magnetic resonance sequence is limited, particularly in the case of a patient as the examination object, wherein large overall ranges of values are to be covered. This is at the expense of the resolution of the material parameters in this range of values. In addition, the magnetic resonance sequence must be selected so that ideally an assignment of magnetic resonance signals to comparison signals can be done over the entire target region. These boundary conditions lead to a restriction of the resolution in the ranges of values for parameter values in quantitative imaging.